1. Field of the Invention
This invention relates generally to location finding and tracking of a satellite by an antenna system. Specifically, this invention relates to satellite antenna acquisition via accurate signal identification for reducing the time for acquisition of a correct satellite.
2. Description of Related Art
Fixed satellite and vehicle-mounted in-motion satellite tracking antennas provide users a means to achieve one-way or two-way communication via satellites. In both fixed and in-motion use, satellite antennas need to be positioned correctly in space in order to receive a signal from a desired satellite. In a fixed satellite application, the set up procedure is performed upon installation and generally does not require satellite re-acquisition unless more than one satellite is desired or natural or environmental effects, such as storms or wildlife, disturb the satellite antenna position. In the in-motion use, the satellite antennas need to be positioned correctly each time they are activated, while they are in-motion and each time they lose the satellite signal due to blockage by objects that naturally appear between the satellite antenna and the satellite as the vehicle moves.
The time it takes to reacquire the satellite signal can range from an annoyance to a technology acceptance-limiting event. In a fixed application, although the occurrence of an incorrectly positioned satellite antenna is infrequent, a trained technician is generally required to position the satellite antenna correctly. Satellite service in this case could be down for hours or days. In in-motion use, satellite reacquisition occurs very frequently with significant, but shorter time intervals to correct positioning.
In conventional satellite antenna acquisition steps, whether manual or automatic, the sky is searched by scanning 360 degrees in azimuth and 20 to 70 degrees in elevation angle. Signal detection during scanning is a two-step process:
1. First, the total received in-band signal power is monitored. As soon as the in-band signal power exceeds a certain threshold level, the antenna is held pointed toward that position in space waiting for a set top box to lock on to the signal and confirm the signal lock.
2. Second, the set top box locks and confirms the signal lock.
The antenna scanning speed and the antenna acquisition time are closely related to how fast the power monitoring in Step 1 can be performed and how fast the confirmation from the set top box in Step 2 can be accomplished. Typically, power monitoring can be performed within a few milliseconds. This means that the speed at which the antenna can scan its beam width through the target can never be faster than a few milliseconds.
Beyond the time and effort required to correctly position the satellite antenna and achieving set top box signal lock (typically about 2-3 seconds), the signal acquisition process is problematic because there are many ways a satellite antenna can experience a false lock. Typical examples of false lock include: locking on a wrong satellite with the same frequency; signal power fluctuation due to noise, inaccuracy in power monitoring and detection circuitry; locking onto the sidelobe of other terrestrial radiators at a closer distance; locking on to noise and locking onto a reflected signal from a nearby structure. Each false lock increases the antenna acquisition time by a few seconds.
The design of the antenna acquisition steps is significantly impacted by the false lock and missed detection effects. If the power-monitoring threshold in Step 1 is set high, false lock probability is reduced. However, there is a higher possibility of missed detection. Each time the missed detection occurs, the antenna must scan through the entire cycle then change the threshold again, then scan again, keep on repeating the process, before returning to the correct position for antenna acquisition. This increases the acquisition time significantly. Lowering the power monitoring threshold in Step 1 leads to frequent false lock, each costing a 2 to 3 second penalty (for Step 2) in antenna acquisition time. Thus, false locks can significantly increase the overall antenna acquisition time.
U.S. Pat. No. 5,585,804 describes the use of electronic compasses to decrease the scanning range, thereby speeding up the satellite signal acquisition. However, electronic compasses can be negatively affected by metal structures or magnetic field from conductors carry current of electrical components in the vehicle. And it is almost impossible to have the resolution of less than 10 degree for automobile application. Which make them unreliable in use with most vehicles and tend to be overly costly for large volume cost sensitive applications.
U.S. Pat. No. 5,828,957 describes an antenna acquisition means by searching for and acquiring a strongest pilot channel, searching for signaling channels on the acquired strongest pilot channel and monitoring the acquired signaling channel instead of beam acquisition of a modulated channel. This system has the limitation that the satellite must transmit pilot tone.
U.S. Pat. No. 6,127,967 describes an antenna acquisition means by searching for and acquiring a beacon signal. This system has the limitation that the desired satellite must transmit a beacon signal.
It is desirable to provide an improved approach to significantly reduce false lock error and the time it takes to acquire the desired satellite at a reasonable cost.